Field of the Invention
The present invention relates to test circuitry for finding flaws in the magnetic media of disks for use in disk drives. More particularly, it relates to an improved implementation of the in-phase/quadrature-phase method of detecting such flaws.
In the manufacture of hard disk drives, raw disks are delivered to the assembly operation for incorporation into disk drives. A raw disk consists of a platter that has the magnetic memory media deposited on one or both surfaces. The magnetic media is deposited using conventional thin film techniques. Because the technology does not currently exist to deposit thin films with no defects, a useful step in a low cost manufacturing process consists of testing the raw disks for flaws before the money is invested to assemble the disk into a drive.
This test step is accomplished in the prior art by writing a sine wave onto the raw disk with a fixture consisting of a mechanism to spin the disk and a head to write the signal. The sine wave is monitored by a "flash" analog to digital converter and an expensive phase locked loop synchronizing circuit. The phase lock loop synchronizes the flash analog to digital converter to sample the peaks of the sine wave. While this approach works well, it is complicated and expensive.
A second prior art approach uses a scheme called the in-phase/quadrature-phase (the "I/Q") method. This method consists of reading the sine wave by converting it to a DC value. However, prior to detection of the DC value, the second method takes advantage of the following relationship: EQU A.sup.2 sin.sup.2 .phi.+A2cos.sup.2 .phi.=A.sup.2 ( 1)
where A is a constant.
That is, for all values of .phi., the sum of the squares of the sine and cos is a constant. With this relationship, it is possible to merely look at dc threshold levels rather than synchronizing an analog to digital converter with the peaks of a sine wave. The I/Q method is generally implemented by dividing the sine wave signal into two signals, shifting the phase of one of the signals by 90.degree. relative to the other signal, squaring the signals and summing them. The resulting dc signal is then passed through a threshold detector. A signal that is greater or less than a predetermined percentage of nominal indicates a super pulse or a missing pulse, respectively.
The I/Q method as it is currently implemented has serious flaws. The current approach uses two passive resistor/capacitor all pass networks to shift the phase of one of the divided signals 90.degree. with respect to the other. However, the two signals are at a 90.degree. degree phase shift at only a fixed frequency. This does not provide the capability to use multiple frequencies appropriate for the drive. In addition, the center point and bandwidth of the 90.degree. phase shifted signals depends on the circuit supplying the signal to the phase shift network and also on the circuit to which the phase shift network supplies the signal, i.e. a multiplier circuit. In addition, the resistors in the r/c network attenuate the signal. Finally, this approach requires exactly the same gain for both signals. But with r/c networks, this is very difficult.